The present invention relates to medical devices and methods. More specifically, the invention relates to intervertebral prosthetic discs and methods of preserving limited motion upon removal of an intervertebral disc.
Back pain takes an enormous toll on the health and productivity of people around the world. According to the American Academy of Orthopedic Surgeons, approximately 80 percent of Americans will experience back pain at some time in their life. In the year 2000, approximately 26 million visits were made to physicians' offices due to back problems in the United States. On any one day, it is estimated that 5% of the working population in America is disabled by back pain.
One common cause of back pain is injury, degeneration and/or dysfunction of one or more intervertebral discs. Intervertebral discs are the soft tissue structures located between each of the thirty-three vertebral bones that make up the vertebral (spinal) column. Essentially, the discs allow the vertebrae to move relative to one another. The vertebral column and discs are vital anatomical structures, in that they form a central axis that supports the head and torso, allow for movement of the back, and protect the spinal cord, which passes through the vertebrae in proximity to the discs.
Discs often become damaged due to wear and tear or acute injury. For example, discs may bulge (herniate), tear, rupture, degenerate or the like. A bulging disc may press against the spinal cord or a nerve exiting the spinal cord, causing “radicular” pain (pain in one or more extremities caused by impingement of a nerve root). Degeneration or other damage to a disc may cause a loss of “disc height,” meaning that the natural space between two vertebrae decreases. Decreased disc height may cause a disc to bulge, facet loads to increase, two vertebrae to rub together in an unnatural way and/or increased pressure on certain parts of the vertebrae and/or nerve roots, thus causing pain. In general, chronic and acute damage to intervertebral discs is a common source of back related pain and loss of mobility.
When one or more damaged intervertebral disc cause a patient pain and discomfort, surgery is often required. Traditionally, surgical procedures for treating intervertebral discs have involved discectomy (partial or total removal of a disc), with or without interbody fusion of the two vertebrae adjacent to the disc. When the disc is partially or completely removed, it is necessary to replace the excised disc material with natural bone or artificial support structures to prevent direct contact between hard bony surfaces of adjacent vertebrae. Oftentimes, pins, rods, screws, cages and/or the like are inserted between the vertebrae to act as support structures to hold the vertebrae and any graft material in place while the bones permanently fuse together.
A more recent alternative to traditional fusion is total disc replacement or TDR. TDR provides the ability to treat disc related pain without fusion provided by bridging bone, by using a movable, implantable, artificial intervertebral disc (or “disc prosthesis”) between two vertebrae. A number of different artificial intervertebral discs are currently being developed. For example, U.S. Patent Application Publication Nos. 2005/0021146, 2005/0021145, and 2006/0025862, which are hereby incorporated by reference in their entirety, describe artificial intervertebral discs with mobile bearing designs. Other examples of intervertebral disc prostheses are the LINK® SB Charité disc (provided by DePuy Spine, Inc.) MOBIDISC® (provided by LDR Medical (www.ldrmedical.fr)), the BRYAN Cervical Disc (provided by Medtronic Sofamor Danek, Inc.), the PRODISC® or PRODISC-C® (from Synthes Stratec, Inc.), the PCM disc (provided by Cervitech, Inc.), and the MAVERICK® disc (provided by Medtronic Sofomor Danek).
A potential drawback of these known disc designs is that the prosthetic disc must be inserted from the anterior side of the patient. The anterior approach can be difficult and may require a vascular surgeon as the prosthetic disc passes near important blood vessels located anterior to the spine. Other currently available intervertebral disc prostheses usually have similar drawbacks, including invasiveness of the surgery and/or surgical skill and complexity.
Another prosthetic approach has been to fuse the vertebrae, for example with transforaminal lumbar interbody fusion (TLIF) surgery or posterior lumbar interbody fusion (PLIF) surgery. These procedures allow the surgery to be performed from the posterior without passing through the abdominal cavity and the associated drawbacks. The TLIF or PLIF approaches involve passing through a much smaller space than an anterior approach and generally require at least partial removal of one or more facet joints to provide enough space for access to the disc space. It is this limitation on space that has until now prevented the design of a successful artificial disc for delivery by a TLIF or PLIF approach.
Therefore, a need exists for an improved disc for preserving motion and maintaining disc spacing between two vertebrae after removal of an intervertebral disc which can be delivered by a TLIF or PLIF approach. Ideally, such improved disc would be introduced in a small configuration and expanded in vivo to a larger configuration.